The reinforced plastic industry has historically used glass fibers in the form of continuous or chopped fibers, strands and rovings to reinforce polymer matrices used in making a wide range of products. In particular, glass fibers, as well as woven and non-woven fabrics, meshes and scrims have been added to thermosetting polymer matrices for use in pultrusion, filament-winding, spray-up, sheet molding and bulk molding operations.
In the pultrusion process, for example, a reinforced composite is formed when a thermosetting polymer or a hot thermoplastic polymer is forced between the fibers of a glass roving as it is pulled through a coating apparatus, profiling and alignment dies. In this manner, the fibers are impregnated with the polymer resin, and the resin and fibers are shaped into the form of the composite. In turn, these impregnated composites are cured by heating continuously. The glass fibers, strands and rovings used in the pultrusion operation are made from molten fiberizable material that is introduced into a bushing or like device. The bushing is equipped with small apertures to allow passage of thin streams of the molten material. As the molten material emerges from the bushing apertures, each stream is attenuated and pulled downward to form a long, continuous fiber. The continuously forming fibers may be gathered into strands for winding. Winding is accomplished by attaching the strands to a mechanism such as a winder or pull wheel, and then winding the fibers onto spools or reels. The rate at which the winder pulls the fiber determines its width, with more rapid attenuation creating thinner fibers of smaller diameter, and a slower speed generating thicker fibers. The rate of attenuation also affects the movement of the filaments through the fiber-forming process, and therefore also affects the amount of any coating material deposited on the surface of the moving fibers. The forming packages or doffs formed by the above-described winding operation are then ready for use in composite-making operations such as pultrusion or filament winding.
Several difficulties have been associated with the use of continuous fibers and the rovings made from these fibers. A major problem with the use of wound rovings is the breakage of the individual fibers during winding, unwinding or handling of the strands. Inter-filament abrasion of the fibers causes them to break, and as a result loose ends are separated from the fiber strands. These loose, broken ends form a roughened layer or fuzz on the surface of the fibers. Fuzz may also develop when fibers break during the weaving process. This fuzz is undesirable because it affects the appearance of the woven product. Breakage of the fibers also results in build-up of fuzz on the contact points and other surfaces of the processing machinery. This fuzz buildup in turn is exacerbated by static electricity. Also, the fuzz often becomes airborne, and thus becomes a source of skin and respiratory irritation to some workers handling the fiber strands. The fuzz may also collect to form tufts or balls of broken fibers, which then jam the processing equipment or fall into the resin baths used for dipping the fiber strands.
It has long been recognized that, in order to reduce fuzz, it is advantageous to provide a light coating known as a size or size coating onto the surface of the glass or carbon fibers after they have been attenuated from the bushing. The size coating is particularly desirable when the fibers are to be used as reinforcing elements in resinous articles. The size coating reduces fuzz and improves processing properties of the fibers such as fiber bundle cohesion, spreadability, fiber smoothness and softness, abrasion resistance and ease in unwinding the fiber bundles. The compositions are further used to make the glass fibers compatible with the polymer matrices to which they are added as a reinforcing component.
The size may be applied to the fibers in an on-line operation immediately after they are formed, or they may be applied off-line to unwound fiber strands that were previously formed and packaged. Preferably, the fibers are wet with the sizing composition as soon as they are formed from the bushing. The application of the composition at this early stage helps to protect the fibers from damage during handling. The sizing is applied by conventional means, then dried onto the surface of the fibers. Heat may be used to dry and/or cure the coating.
Generally, sizing agents can be divided into two types, the solvent type and the emulsion type. Non-aqueous solvent type size compositions include a low-boiling organic solvent combined with the ingredients to form a solution that is applied to the fibers. In emulsion sizing agents, the resin and other ingredients are dispersed in water to form an emulsion, which is then applied to the filaments. The filaments are dried to provide a uniform coating of the sizing agent. Chemical sizing compositions have traditionally been aqueous solutions, foams or gel compositions containing a film-forming polymer, coupling or keying agent, and lubricant.
Typically, the focus in the art has been concentrated on developing sizing compositions that either maximize the amount of coating on the fibers to reduce fuzz; or on eliminating undesirable features of coated roving, such as tackiness or flaking. As a result, the sizing compositions that are usually used in the art to reduce fuzz formation include high levels of lubricants or other ingredients. Coatings as exemplified by U.S. Pat. Nos. 5,393,335, 5,605,757, 5,665,470, 5,334,639, 5,286,562, 4,795,678, 4,762,751, relate to sizing compositions for treating glass fibers having one or more ingredients such as lubricants, emulsifiers, humectants, coupling agents or natural or synthetic polymers. However, the high levels of lubricants traditionally used in these size coatings routinely result in over-lubrication of the strands or rovings. When these over-lubricated rovings are wound, the forming packages become unstable because of slippage of the strands. The rovings may also be squeezed together during the packaging, thus changing the height of the doff. As a result, the formed packages are not of uniform height, and they cannot be stacked or palleted for efficient storage. Rather, stacking pallets with unevenly sized packages may cause the pallets to collapse, thereby jeopardizing worker safety. The lack of uniform size may also result in the packages not being able to fit onto apparatus used to mold or form composites. Use of higher amounts of lubricants has also increased the costs associated with the sizing operation.
Another problem related to the use of sizing compositions is incompatibility between the sizing composition and the polymer matrix used to form the composites. The art has attempted in several ways to solve the problem of incompatibility between the fibers and the polymer composite material into which they are implanted, including the development of compositions containing curing or coupling agents. However, there remains a recognized need for a coupling agent that facilitates intimate bonding between the glass fibers and the polymer matrix.
There exists then a need for a sizing composition that will provide an effective coating to reduce fuzz, while improving rather than decreasing the stability of the roving package. Further, a sizing composition that yields an effective coating, but at the same time requires a low concentration of ingredients is desirable because waste is minimized.